It is conventional in offshore seismic operations to tow a long marine streamer cable behind an exploration vessel at a controlled depth. The streamer cable contains hydrophones that detect pressure variations produced by reflected seismic waves. The reflected seismic waves are the result of a seismic wave travelling through the earth and being reflected back to the surface. The seismic waves, of course, are produced by a seismic source, such as an air gun. The signal produced by the streamer cable will have several frequencies of reduced amplitude as a result of the surface reflected wave or ghost.
The ghosts are produced by upgoing reflected wavefront striking a horizontal boundary, i.e., the ocean surface and being reflected back to the hydrophone located below the surface. As the wavefront strikes the boundary, its phase or polarity will be reversed because of the negative reflection coefficient of the air/water interface. Thus, if the distance between the hydrophone and the surface and back to the hydrophone is equal to one wave length of a particular frequency, or a multiple therof, the amplitude of that frequency and its multiples will be reduced.
It has been appreciated as described in U.S. Pat. No. 3,290,645 that the frequencies cancelled by the above phenomena could be replaced by the use of geophones to record these frequencies. Geophones respond to the particle velocity of wave motion rather than the pressure. The wave of particle velocity will have a positive reflection coefficient at the air/water interface. Thus, if the distance between geophone and surface and back to the geophone is equal to one wave length of a particular frequency, or a multiple thereof, the amplitude of the frequency and its multiples will be increased. Thus, the geophone signal will replace the frequencies that are removed in the hydrophone signals. Accelerometers could be used instead of geophones for this purpose. The accelerometer signal could be integrated electronically, or numerically in a computer after recording, to produce an estimate of the velocity signal. U.S. Pat. No. 3,281,768 describes the use of accelerometers and hydrophones in a marine streamer cable. The patent also describes integrating the accelerometer signal and combining it with the pressure signal produced by the hydrophone. Theoretically, this would solve the problem of missing frequencies but in practice the noise introduced by the slip rings shown in the patent is amplified by integration. The resulting signal-to-noise ratio makes it difficult if not impossible to combine the two signals. While it has been appreciated that geophones could be used to replace these missing frequencies, it is difficult to obtain a geophone or an accelerometer which will operate in a marine seismic environment. Geophones and accelerometers require that their axes be very carefully aligned in certain directions in order to obtain proper response. It can be readily appreciated that it is difficult to maintain the proper alignment of the axes in a marine streamer towed behind a vessel because the cable undergoes continuous and random rotation around its longitudinal axis. In addition, the accelerometers may be sensitive to pressure and thus respond to the pressure produced by the reflected seismic waves. The response to the pressure variations results in noise that tends to mask or cancel the true accelerometer signal.